In drilling an oil well, the drill string normally incorporates several drill collars and stabilizers. The drill pipe, itself, normally does not touch the wall of the drilled hole. The stabilizers, one or more, function to clear the pipe and drill collars from the wall. The stabilizer, however, is intended to touch the wall of the drilled hole to align the hole as it is formed by the drill bit. The stabilizer normally includes lengthwise splines mounted on its exterior which may be straight or helical about the stabilizer. The splines keep the drill string aligned so that the hole is drilled relatively straight, the hole being drilled to a diameter determined by the drill bit. The splines on the stabilizer normally contact the wall of the drilled hole. Because of this contact, the splines eventually wear down, and this necessitates replacement of the stabilizer from time to time.
A very expensive solution to wearing of stabilizers has been devised in the past. The entire stabilizer is formed from a single bar or heavy wall tubing of hardened alloy steel such as 4140-4145 steel. The external surface is milled away to leave the splines along the external surface of the stabilizer. This is a very expensive procedure in that a tremendous amount of machine time is required for the milling operation, and the milling operation, itself, is slow because the bar is a relatively hard alloy steel. It has the advantage, however, of a very strong root connection between the spline and body. This is particularly valuable because it prevents fracture along the root and separation of the spline from the stabilizer body.
Another form of stabilizer is obtained by utilizing a spline which is manufactured separate from the stabilizer body to decrease machining costs. The spline is later welded to the stabilizer body, achieving more than an adequate financial tradeoff in the cost of fabrication.
Even with the stabilizer formed from a bar or heavy wall tubing of heat treated alloy steel, it eventually becomes necessary to weld hard surfacing (wear resistant) material across the spline. Even with the best grade of hard surfacing, the spline will wear away. One solution begins with the step of drilling holes in the spline and pressing cylindrical or serrated cylindrical inserts of a hardened material into the holes. One material is tungsten carbide particles in an alloy matrix support metal formed into a suitable insert. This adds tremendous wear life to the stabilizer.
When the tungsten carbide inserts are pressed into predrilled holes, even when they are assembled with an interference fit, the inserts can still sometimes come out, typically occurring in the following circumstances. After many hours of usage, the splines and the protective tungsten carbide inserts are worn down, and the wear manifests itself in a reduction of height on the outer diameter of the stabilizer. One manner of correction of this problem is to weld an overlay of hard surfacing onto the exposed or outer face of the spline, even over the inserts. One such overlay material utilizes welding material with tungsten carbide particles in it so that the built-up layer on top of the spline is abrasive resistant. This is accomplished in either a welding or bonding process which requires heating of the spline. The alloy supported tungsten carbide placed on the face is bonded to the spline in a procedure accompanied by heating of the spline. The difficulty with this is that the tremendous amount of heat applied to the spline creates severe problems. The heating does not alter the tungsten carbide insert, itself, but it creates forces and stresses within the spline which may cause the spline to expel the tungsten carbide insert. One explanation is that the tungsten carbide inserts, when pressed into a drilled hole, sometimes trap moisture and air under the insert. When the spline is heated in the refacing process, the air, water and other gases expand and pressurizes the insert to an extent that it is expelled. In some instances, inserts have been fired from the drilled holes in the fashion of a rifle bullet with equal danger. This makes the process of redressing the face of the spline dangerous for splines with pressed-in inserts.